Thursday, 1 November 2007

How linear is Goldie?

A non-linear position translates a normal distribution of market returns into a non-normal distribution of P/Ls. It used to be reasonable to assume that trading revenues were normally distributed. For instance, here is Goldman from Q3 2004 (picked up via Alea):

Now, though, some of the broker dealers are strongly non-linear. Here's Goldman Q3 2007:

Morgan Stanley for the same period is similarly non-normal:

There's nothing inherently wrong with this, but it does mean that any old style investment model (CAPM, for instance) which relies on normal returns won't deal with stocks like this correctly, and concepts like beta with the market are less meaningful.

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